The present invention relates to the production of ultrafiltration, hyperfiltration or demineralization elements having simultaneously good mechanical properties, particularly a good rigidity and pressure resistance, and good physicochemical properties, particularly a high permeability, small size pores with a narrow distribution around the mean value and optionally a marked ionic character. Hitherto elements of this type have been produced either from inorganic materials, or from organic materials.
When use is made of inorganic materials, it is possible to produce elements having good mechanical characteristics and interesting physicochemical properties with regards to the permeability, the dimensions of the pores and their distribution, by depositing on a macroporous mineral support a microporous mineral layer, which can e.g. be obtained from a peptized gel, as described in French patent No. 2 550 953 filed on 12.4.1977 in the name of the Commissariat a l'Energie Atomique.
As a result of the properties of the macroporous support, said elements have a good rigidity and pressure resistance and the production of the microporous layer from a peptized gel makes it possible to obtain the desired distribution and pore sizes, as well as a high permeability. However, such elements cannot have a marked ionic character.
When use is made of organic materials, the standard practice is to produce organic polymer membranes, e.g. by casting from a solution of the polymer in an appropriate solvent, followed by evaporation of the solvent. The polymers which are more particularly used are cellulose esters and it is possible to obtain therefrom semipermeable membranes suitable for the demineralization of salt water, such as sea or ocean water and as described in U.S. Pat. No. 3 133 132, filed on 29.11.1960 in the name of S. Loeb et al.
These organic membranes have very interesting physicochemical properties for separations by ultrafiltration or demineralization, particularly due to the fact that it is possible to functionalize them by introducing into the polymer anionic or cationic groups, which is advantageous when they are used for the demineralization of not very concentrated saline solutions and the ultrafiltration of colloids. However, these organic membranes suffer from the disadvantage of being mechanically fragile and of consequently requiring the use of reinforcing and supporting structures. For example, it is possible to use woven or non-woven supports made from a polymer compatible with the organic polymer constituting the membrane. It is also possible to place them on perforated tubular metal supports. However, this requires supplementary fitting operations for the membranes, which is prejudicial to the cost of the installations. They can also be produced in the total thickness of a porous support by impregnating the support with a solution, followed by evaporation of the solvent and as described in EP-A-139 806, DE-A-2 052 236, FR-A-425 563 and GB-A-1 197 572.
Thus, the membranes formed in the porous support have a thickness at least equal to that of the porous support and under these conditions a high permeability cannot be obtained.
Consideration has also been given to producing filters having a porous ceramic support, on which is placed a semipermeable organic polymer microporous membrane, by blocking the pores during the fitting of the organic membrane and then dissolving the pore-blocking material following the fitting of the organic membrane, as described in Japanese patent No. 59/206008, filed under No. 081562 on 10.5.1983 by the TDK Corp.
However, this manufacturing procedure does not make it possible to obtain satisfactory results. Thus, the active layer has an imperfect cohesion with the support and is therefore fragile. Due to its limited thickness and position on the surface, it has a significant fragility or sensitivity to scratches/grooves and impacts, as well as to the tangential force of the flow of fluids to be treated.
Moreover, certain technical difficulties are encountered in finding blocking agents which can be dissolved at the end of the operation without harming the active organic polymer layer.